A drill bit is a rotating cutting tool for drilling steel products and the like. FIG. 1 shows an exemplary structure of a conventional twist drill bit. The twist drill bit is formed by a cutting portion 1 which is applied to drilling, and a shank 2 which is adapted to discharge chips and is held in a chuck etc. of a cutting machine such as a drilling machine.
In general, materials for drill bits are prepared from high-speed steel and cemented carbide. The high-speed steel, which is excellent in toughness but inferior in wear resistance, is improper for high-speed cutting. On the other hand, cemented carbide, which is excellent in wear resistance and tool accuracy characteristics but brittle, may cause breakage when the same is applied in a machine tool having a low rigidity, for example.
In order to improve such materials, there has been proposed a structure attained by coating a cutting portion of high-speed steel with hard TiN, a structure attained by brazing a cutting portion of cemented carbide, and the like.
In recent years, there has further been proposed a structure attained by brazing different cemented carbide materials (P30 and D30) for improving the wear resistance and toughness as disclosed in Japanese Utility Model Laying-Open No. 58-143115 (1983), or by metallurgically integrating/connecting such different materials as disclosed in Japanese Utility Model Publication No. 62-46489 (1987). A drill bit having a double structure with central and outer peripheral portions has been made of different cemented carbide materials in consideration of the differences between the characteristics required for such drill bit portions, as disclosed in Japanese Patent Laying-Open No. 62-218010 (1987). A method of attaining such a double structure by injection molding has been disclosed in Japanese Patent Laying-Open No. 63-38501 or 38502 (1988). There has also been proposed a structure attained by preparing a material for a drill bit from cermet Ti-base cemented carbide in order to improve the adhesion resistance, as disclosed in Japanese Patent Laying-Open No. 62-292307 (1987).
A cutting portion and a shank of a drill bit are exposed to different load conditions during drilling. Therefore, different material characteristics are required for the respective parts or portions of such a drill bit. For example, wear resistance and adhesion resistance are required for a tip of the cutting portion, while toughness for maintaining strength of the tool is required for the shank. As to the tip of the cutting portion, different characteristics are required for central and outer peripheral portions thereof, since these portions are driven at different cutting speeds.
When a drill bit with a coated cutting portion for meeting the complicated requirements for the material characteristics, is resharpened for general use, the coating layer is inevitably separated at least from a front flank side, and hence most of the coating effect is lost.
On the other hand, a drill bit which is formed by brazing cemented carbide to its cutting portion, cannot be used for deep hole drilling of a hard to cut material. If the shank is made of steel, a significant difference between the thermal expansion coefficients of the steel and of the cemented carbide forming the cutting portion, exists whereby splitting or cracking may easily be caused during brazing.
In recent years, a cemented carbide material for the shank of a drill bit, has been brought into a coarse grain or high binder phase state, in order to improve the toughness of the shank. However, the strength of the material is reduced by distortion of the elastic limit. Hence, the shank has a tendency to break during perforating, or due to vibration of a workpiece, an unstable rotation of a cutting machine or the like.
While a drill bit formed by inseparably and integrally connecting the cutting portion and the shank, can be repeatedly used by resharpening the cutting portion after a prescribed operating time, the number of such resharpenings is limited, and production costs are increased. Further, sharpness and the tool life of such bits are non-uniform depending on conditions under which the resharpening is performed. In addition, it is necessary to successively and correctly ascertain the length of the resharpened drill bit if the bit is used in a numerically controlled and automated cutting machine tool. Thus, the length of the drill bit must be measured by a complicated operation every time the same has been resharpened.